This invention relates to magnetic bridges and more particularly to improved magnetic bridge devices providing electromagnetic evaluation of various samples and materials by measurement of the complex magnitude and phase components of an induced magnetic flux.
Magnetic bridges driven by time-varying magnetic fields (U.S. Pat. No. 3,379,969 of O. H. Zinke) can be controlled through combinations of resistances and reactances coupled to the bridge arms (U.S. Pat. No. 3,534,253 of O. H. Zinke) and used to measure conducting and/or magnetic characteristics of samples either inserted directly in the gaps in the bridge arms or inserted in the fringing fields of the gaps. For maximum sensitivity, samples have to be placed directly in the gaps in order to intersect the maximum amount of time-varying flux. If, for reasons of convenience, samples are placed in the fringing flux, the sensitivity of the bridge to the characteristics of the sample changes in the approximate ratio of the fringing flux intersected by the sample to the total flux in the gap. Thus, in the magnetic bridges described in the above two patents, the most efficient use occurs when the sample is placed directly in the gap in a bridge arm. Under these circumstances, the size of the sample which can be inspected is restricted by the width of the gap which can be used and the length of the bridge arm. For example, if an aircraft wing were to be inspected for fatigue in the aluminum skin, the bridge arms would have to be about half the width of the aircraft wing, and the gap in the bridge would have to be larger than the thickness of the wing. Such a wide gap would greatly reduce the time-varying magnetic flux because of the tendency of flux to spread as it leaves a magnetic pole (the face of the gaps). Further, and possibly more important, the eddy currents induced in the sample under such circumstances may not have paths which are appropriate to the detection of the characteristic sought. In the above example, the eddy-current paths could be of the order of the sample, the wing, while the characteristic being searched for would be small cracks appearing near the rivets in the wing. The sensitivity of the bridge would be further reduced by the ratio of the volume of the crack to the volume of the whole wing.
A solution to the problem above described, as herein set forth, involves modification of the magnetic bridge devices of the above patents. This modification, involving use of a conductive metal insert in a gap of the bridge to serve as means for enhancing a fringing flux exteriorly of the gap, is termed "single-side" detection or "single-side" technique because the fringing flux produced by the bridge can be used to inspect the sample from one side of the sample. Further, the modification reduces the volume of the sample being scanned to fractions of a cubic inch. It is possible to detect cracks near rivets in aircraft wings with a bridge which is less than a cubic inch in volume through the use of this modification. The effect of this modification, further, is to move all the flux in a magnetic gap carrying a time-varying magnetic field completely out of the gap and into the fringe field.
Thus, single side detection utilizing magnetic bridge devices according to the invention can be carried out for nondestructive evaluation of variously configured structures and materials of ferrous or non-ferrous metal. The evaluation will reveal cracks, pre-crack or residual stress, surface flaws and volume voids or anisotropies.
Use of the invention is not limited to the evaluation of materials along planar surfaces. A specific modification of the above-described magnetic bridge devices, as disclosed in said patents, provides a device embodiment which may be inserted within a tubular structure, such as pipe, for detecting flaws within the pipe or other structure. In certain critical applications of pipe or other tubing such as used, for example, for carrying high pressure steam within nuclear power plants and for use in heat exchangers in such plants, it is important to locate flaws such as within the walls of iron pipe. A device of the present invention is inserted solely within such pipe. The device, as configured for this use, includes driving and sensing legs which define respective axes which intersect the symmetrical axis of elongation of the structure and which are, themselves, mutually perpendicular, being preferably spaced apart along the axis of elongation to introduce within the walls of the structure four effective arms of the magnetic bridge. Magnetic flux is produced in the driver leg of the bridge, and the design causes the time-varying magnetic flux thus produced to divide into quadrants within the pipe walls. These quandrantal components are compared to each other by a sensing or signal leg of the bridge, and if the flux in the quadrants does not balance, a time-varying flux is induced in the sensing leg. It is sensed by a coil wound about the sensing leg, and the resultant electrical signal is analyzed for amplitude and phase of the fundamental and harmonic signals which appear, the phase being compared to the electrical signal which produces the flux in the driver leg of the bridge. The device is designed to produce components of magnetic flux which extend both circumferentially and axially within the walls of the pipe.
According to a further embodiment of the invention, a magnetic bridge device utilizing "single-side" technique is employed as an accelerometer, herein termed bridge-accelerometer by using a magnetic bridge to sense the position of an elastically suspended mass.
Accordingly, among the several objects of the inventions may be noted the provision of magnetic bridge devices of various configurations for carrying out non-destructive evaluation of ferrous and non-ferrous samples and structures, such as to detect cracks, pre-crack stresses, surface flaws, volume voids, anisotropies and other anomalies therein, regardless of the shape or size of the sample or structure, such as for the purpose of coin testing, pipe testing, surface testing, aircraft structure evaluation, and many other evaluative purposes; and which can detect hysteresis loop "signatures" such as can predictably and accurately reveal the characteristics.
It is further an object of the invention to provide such magnetic bridge devices which can be utilized for other evaluative and measurement schemes, including measurement of temperture changes of samples of ferrous or nonferrous metals regardless of size, shape or condition of motion of the samples; which can detect and/or measure thicknesses of laminants on ferrous or non-ferrous metals (such as coins and other clad or laminated metals, whether the laminants or surface coatings thereof are metallic or non-metallic; which can be utilized for measurement of the dimensions of samples of ferrous or nonferrous metals and of indentations or imprints in such samples, such as, for example, on coinage or on ruled objects, such as metal rulers.
Among other objects of the invention are to provide magnetic bridge devices which can be utilized for the measurement of distance or angular displacements with devices such as linear or angular transducers, such as for vibration detection or for motion detectors, and specifically, as an accelerometer. It is also an object to provide magnetic bridge devices which can be utilized for the detection of metallic objects or variations in displacements of metallic objects such as on assembly lines or in intrusion alarm systems.
Other objects and features will be in part apparent and in part pointed out hereinbelow.